(a) Field of the Invention
The present invention is related to a multi-warm forming device and a forming method thereof. More particularly, the present invention relates to a multi-warm forming device of which one mold set performs warm forming and blow-forming having different temperature conditions from each other so as to produce a product having a complicated forming shape and a large forming depth, and a forming method thereof.
(b) Description of the Related Art
Generally, a warm forming process has been developed to form a magnesium alloy sheet of a lightweight material of which a density of the metal structure is lower than that of an aluminum alloy sheet, and has recently been under development and is being applied by demand of various press forming methods so as to apply a magnesium alloy sheet to a vehicle body for the purpose of producing a lightweight and high strength vehicle body in America.
That is, a warm forming method is performed at an intermediate temperature range between cold forming and hot forming temperatures, wherein a sheet receives heat energy from a high temperature mold that is heated by a heat source, and press forming is performed under conditions such that a yield strength is reduced and an elongation rate is improved.
The magnesium alloy sheet to which the warm forming method is applied has an HCP (hexagonal closed packed) crystal lattice structure, so it is difficult to apply a press forming method thereto at room temperature due to the crystal structure, and formability is quickly improved by a characteristic that a non-basal plane slip system is activated in a high temperature area (of higher than 200° C.).
However, the magnesium has high specific strength, and it can be light in weight at 30% lighter than an aluminum alloy, but it is disadvantageous in an aspect of cost, corrosion, formability, and welding characteristics compared to other materials such as an aluminum alloy.
Particularly, in a case that a product having a complicated shape or a product having a large forming depth is produced, there are drawbacks that the number of processes and the number of components are increased due to limitations of formability, forming cost is increased, and productivity is deteriorated.
Meanwhile, an aluminum alloy is disadvantageous in terms of weight compared to a magnesium alloy and is advantageous in an aspect of material cost and formability, and thus a die casting process has been used therewith to produce a product of which a forming shape is complicated and a forming depth is large.
However, the die casting method injects a molten metal of an aluminum alloy into a die to perform casting, wherein the facility cost is high for mass production, the number of the processes is larger, and there is a drawback in terms of productivity.
Therefore, a new forming method that uses super-plasticity as a physical characteristic of an aluminum alloy has recently attracted attention, wherein the super-plasticity is a characteristic that the material shows extreme ductility without local shrinkage when the material is deformed under a specific temperature condition.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.